How can I remove “gonna” so that it never appears as a suggestion without having to write a rule specifically for the token “go.”

I don’t think there’s a way to remove it only from this rule’s suggestions. But as it’s colloquial, the question is whether it should be suggested at all. If not, it can be removed by adding it to removed.txt.

Adding “gonna” to the removed.txt file (gonna (tab) go (tab) VBG) has the following effect.
In the desktop Jar, CTRL + T reveals that “gonna” is now tagged with “null” but the suggestion continues for match no=“8” postag=“VBG” as “going, gonna”.

I don’t ever want to suggest “gonna” as the VBG of “go”. Is there somewhere else I can remove it?

Adding “gonna” to the removed.txt file (gonna (tab) go (tab) VBG) has the following effect.

Sorry, I forgot that the synthesizer (which finds all the forms of a lemma) does not consider remove.txt. What you could try is this (not tested): add gonna to prohibited.txt and add suppress_misspelled="yes" to your match element.

As you can see, the LT system returns the first two items in the suggestion but leaves out the prohibited word. This makes for a ponderous feedback message. One could say that I am overloading the error detection rule by offering three forms in the suggestion. I might have to reconsider my formulation of this rule. The reason why I have made it this way is to illustrate the parallel series using the learner’s own sentence.